JP6690158B2 - Inner surface inspection device and positioning method - Google Patents

Description

  The present invention relates to an inner surface inspection device and a positioning method.

2. Description of the Related Art In a manufacturing process of an engine mounted on an automobile, an inner surface inspection device that inspects an inner peripheral surface of a cylinder for defects such as scratches and cracks is used. A conventional inner surface inspection apparatus is described in, for example, Japanese Patent Laid-Open No. 2009-69374. The inner surface inspection apparatus of the publication has a transparent columnar body that is inserted inside the hole, an illumination irradiation unit that irradiates the columnar body with light, and an imaging unit. At the tip of the columnar body, a conical portion that is hollowed out in a conical shape is provided. The imaging means inspects the inner surface of the hole by taking an image of the cone.
JP, 2009-69374, A

  In this type of inner surface inspection apparatus, it is necessary to position the rod lens so that the central axis of the transparent cylindrical body (hereinafter referred to as "rod lens") matches the central axis of the hole to be inspected. If the central axis of the rod lens is arranged at a position deviated from the central axis of the hole, the image to be acquired may be biased or the image of the portion that should be visible may become unclear. As a result, the defect to be detected may be overlooked.

  Conventionally, an operator in charge of the inspection process finely adjusts the position of the rod lens by manually operating the inner surface inspection device while visually confirming the position of the rod lens with respect to the hole. However, in such a manual operation, the positioning accuracy varies depending on the operator. Further, there is a possibility that the rod lens may collide with the inner peripheral surface of the hole and the rod lens may be damaged due to an operation error of the operator.

  An object of the present invention is to provide an inner surface inspection apparatus and a positioning method capable of automatically performing the work of positioning the rod lens so that the central axis of the rod lens and the central axis of the surface to be inspected coincide with each other. Is.

An exemplary first invention of the present application is an inner surface inspection apparatus for inspecting a state of a surface to be inspected, which is a cylindrical inner peripheral surface, and extends in the axial direction along an optical axis which is a central axis, A cylindrical or cylindrical rod lens inserted inside the surface, a moving mechanism for moving the rod lens in the axial direction and a direction orthogonal to the axial direction, and an image through the rod lens. An image pickup unit for acquiring and a control unit for controlling the moving mechanism, wherein the control unit inserts the rod lens inside the surface to be inspected, and the rod lens The brightness around the optical axis in the image acquired by the imaging unit in a state in which the rod lens is inserted inside the surface to be inspected and the second movement instruction unit that moves in a direction orthogonal to the axial direction. Based on the distribution, With the an estimation unit that estimates a relative position of the target axis is the center axis of the test surface that, on the basis of the relative position, and a third movement instruction unit for moving the rod lens into the target shaft side.

A second exemplary invention of the present application positions a cylindrical or cylindrical rod lens that extends in the axial direction along an optical axis that is a central axis, inside a surface to be inspected that is a cylindrical inner peripheral surface. A positioning method, comprising: a) inserting the rod lens inside the surface to be inspected; and b) inserting the rod lens inside the surface to be inspected. Moving in a direction orthogonal to the direction, and measuring the luminance distribution around the optical axis in the image acquired via the rod lens, and c) the luminance distribution around the optical axis based on the luminance distribution. The method includes the steps of estimating the relative position of a target axis that is the central axis of the inspection surface, and d) moving the rod lens to the target axis side based on the relative position.

  According to the exemplary first invention and second invention of the present application, the work of positioning the rod lens can be automatically performed so that the central axis of the rod lens and the central axis of the surface to be inspected coincide with each other. .

FIG. 1 is a perspective view of the inner surface inspection apparatus. FIG. 2 is a sectional view of the inner surface inspection apparatus. FIG. 3 is a functional block diagram conceptually showing the functions in the control unit related to the positioning of the rod lens. FIG. 4 is a flowchart showing the flow of processing when positioning the rod lens. FIG. 5 is a diagram showing changes in the position of the rod lens inside the surface to be inspected. FIG. 6 is a diagram showing changes in the position of the rod lens inside the surface to be inspected. FIG. 7 is a diagram showing changes in the position of the rod lens inside the surface to be inspected. FIG. 8 is a diagram showing changes in the position of the rod lens inside the surface to be inspected. FIG. 9 is a diagram showing an example of an image acquired via the rod lens before positioning. FIG. 10 is a diagram showing an example of an image acquired through the rod lens after positioning. FIG. 11 is a graph showing an example of the luminance distribution. FIG. 12 is a diagram showing changes in the position of the rod lens inside the surface to be inspected.

  Hereinafter, exemplary embodiments of the present invention will be described with reference to the drawings. In the present application, a direction parallel to the central axis of the rod lens is “axial direction”, a direction orthogonal to the central axis of the rod lens is “radial direction”, and a direction along an arc centered on the central axis of the rod lens is “ Circumferential direction ". Further, in the present application, the shape and the positional relationship of each part will be described with the front end side of the rod lens along the axial direction as “axial front” and the imaging unit side as “axial rear”.

<1. Overall configuration of internal inspection equipment>
FIG. 1 is a perspective view of an inner surface inspection apparatus 1 according to an embodiment of the present invention. FIG. 2 is a sectional view of the inner surface inspection apparatus 1. The inner surface inspection apparatus 1 is an apparatus that images the inner peripheral surface of a through hole or a concave portion and inspects whether the state of the inner peripheral surface is abnormal. The inner surface inspection device 1 is used, for example, to inspect an inner peripheral surface of a cylinder mounted on an engine of an automobile. However, the inner surface inspection apparatus of the present invention may inspect an object to be inspected other than the cylinder.

  As shown in FIGS. 1 and 2, the inner surface inspection apparatus 1 of the present embodiment has a rod lens 10, a light source 20, an imaging unit 30, a casing 40, a moving mechanism 50, and a control unit 60.

  The rod lens 10 is a cylindrical transparent member that is inserted inside the surface 100 to be inspected, which is a cylindrical inner peripheral surface. As a material of the rod lens 10, for example, glass or acrylic is used. The rod lens 10 extends in the axial direction along the optical axis 91 which is the central axis. A base end surface 11, which is an end surface on the axial rear side of the rod lens 10, is cut into a plane shape perpendicular to the optical axis 91, and the surface thereof is polished. On the other hand, the front end surface 12, which is the end surface on the front side in the axial direction of the rod lens 10, is hollowed out in a conical shape of 45 ° with respect to the optical axis 91, and the surface is polished. The tip surface 12 of the rod lens 10 is mirror-coated.

  The light source 20 is a light emitting body that irradiates the rod lens 10 with light. The light source 20 is arranged axially rearward of the base end surface 11 of the rod lens 10. The light source 20 of the present embodiment is formed in an annular shape and has a circular hole 21 in the center thereof. The light source 20 is arranged coaxially with the optical axis 91 of the rod lens 10. The light emitted from the light source 20 enters the rod lens 10 via the base end surface 11 of the rod lens 10. Further, the light is reflected radially outward by the conical tip surface 12 of the rod lens 10 and is irradiated on the cylindrical surface 100 to be inspected.

  The imaging unit 30 is means for acquiring an image of the surface 100 to be inspected via the rod lens 10. As the image pickup unit 30, for example, a digital camera having an image pickup device such as CCD or CMOS is used. The imaging unit 30 is arranged on the axial rear side of the light source 20. The light emitted from the light source 20 and irradiated on the surface 100 to be inspected again enters the rod lens 10, is reflected by the conical tip surface 12, and travels axially rearward. Then, the reflected light passes through the base end surface 11 of the rod lens 10 and the circular hole 21 of the light source 20 and enters the imaging unit 30. Thereby, the imaging unit 30 can acquire an image of the surface 100 to be inspected.

  The casing 40 is a housing that holds the rod lens 10, the light source 20, and the imaging unit 30. The base end portion of the rod lens 10 is inserted into a through hole 41 provided in the front surface of the casing 40 in the axial direction, and is fixed to the casing 40. The light source 20 is arranged inside the casing 40. The imaging unit 30 is fixed to the axially rear end of the casing 40. The relative positional relationship among the rod lens 10, the light source 20, and the imaging unit 30 is fixed by the casing 40.

  The moving mechanism 50 is a mechanism that moves the rod lens 10, the light source 20, the imaging unit 30, and the casing 40. The moving mechanism 50 has a stage 51 that supports the casing 40. The moving mechanism 50 transmits a driving force generated from a driving source such as a motor to the stage 51 via a power transmission mechanism such as a ball screw. As a result, the stage 51 can be moved in the three directions of the axial direction, the horizontal direction orthogonal to the axial direction, and the vertical direction orthogonal to the axial direction. When the stage 51 moves, the rod lens 10, the light source 20, the imaging unit 30, and the casing 40 move together with the stage 51.

  The control unit 60 is a unit for controlling the operation of each unit in the inner surface inspection apparatus 1. The control unit 60 is composed of a personal computer having an arithmetic processing unit such as a CPU, a memory such as a RAM, and a storage unit such as a hard disk drive. Further, as indicated by an arrow in FIG. 1, the control unit 60 is electrically connected to each of the light source 20, the imaging unit 30, and the moving mechanism 50 described above. The control unit 60 positions the rod lens 10 inside the surface 100 to be inspected by controlling the operation of the moving mechanism 50. In addition, the control unit 60 acquires the image of the surface 100 to be inspected captured by the imaging unit 30 while causing the light source 20 to emit light. Then, by analyzing the obtained image, it is inspected whether there is any abnormality in the state of the surface 100 to be inspected.

  Further, as shown in FIG. 1, a display unit 601 is connected to the control unit 60. The display unit 601 displays the image acquired by the imaging unit 30 and various information related to the inspection on the screen. For the display unit 601, for example, a liquid crystal display is used. The user of the inner surface inspection apparatus 1 can check the images and information displayed on the display unit 601 at any time.

<2. Positioning of rod lens>
In this inner surface inspection apparatus 1, the rod lens 10 is positioned inside the surface 100 to be inspected at the start of the inspection. The operation at the time of positioning will be described below.

  FIG. 3 is a functional block diagram conceptually showing the functions in the control unit 60 related to the positioning of the rod lens 10. As illustrated in FIG. 3, the control unit 60 according to the present embodiment includes an image input unit 61, a determination unit 62, an estimation unit 63, a first movement instruction unit 64, a second movement instruction unit 65, and a third movement instruction unit 66. Have. The estimation unit 63 includes a brightness distribution acquisition unit 631, a virtual straight line calculation unit 632, and a relative position calculation unit 633. The functions of these units are realized by the computer as the control unit 60 operating according to the installed computer program.

  FIG. 4 is a flowchart showing the flow of processing when positioning the rod lens 10. 5 to 8 are diagrams showing changes in the position of the rod lens 10 inside the surface 100 to be inspected. 5 to 8 show cross sections orthogonal to the optical axis 91.

  When positioning the rod lens 10, first, the first movement instruction section 64 in the control section 60 transmits a control signal to the movement mechanism 50. Thus, the moving mechanism 50 is operated to insert the rod lens 10 inside the surface 100 to be inspected (step S1). At this time, the moving mechanism 50 moves the rod lens 10 toward the position of the central axis of the surface 100 to be inspected (hereinafter referred to as “target axis 92”) based on the position information input in advance. However, in this step S1, the optical axis 91 of the rod lens 10 after movement does not exactly coincide with the target axis 92 due to a drive error of the movement mechanism 50. That is, as shown in FIG. 5, the optical axis 91 of the rod lens 10 is arranged at a position displaced from the target axis 92.

  When the rod lens 10 is inserted inside the surface 100 to be inspected, the control unit 60 causes the light source 20 to emit light and causes the imaging unit 30 to perform imaging. The image acquired by photographing is input to the image input unit 61 in the control unit 60 (step S2).

  Next, the brightness distribution acquisition unit 631 in the control unit 60 measures the brightness distribution around the optical axis 91 in the acquired image (step S3). FIG. 9 is a diagram showing an example of an image first acquired after the rod lens 10 is inserted inside the surface 100 to be inspected. When the optical axis 91 and the target axis 92 do not coincide with each other, a change in brightness clearly occurs around the optical axis 91 in the image, as shown in FIG. Specifically, high-luminance portions and low-luminance portions alternate around the optical axis 91 at approximately 90 ° intervals in the circumferential direction. In particular, the brightness in the direction from the optical axis 91 to the target axis 92 becomes low.

  In step S3, the brightness distribution acquisition unit 631 measures the brightness distribution in the image along the measurement circle 93 centered on the optical axis 91. The diameter of the measurement circle 93 may be set to a size that makes it easy to capture changes in brightness. For example, a change in luminance is likely to occur in a portion slightly inward of the outer peripheral surface of the rod lens 10 in the radial direction. Therefore, the measurement circle 93 may be, for example, a circle that is radially inward of the outer peripheral surface of the rod lens 10 and closer to the outer peripheral surface of the rod lens 10 than the optical axis 91.

  FIG. 11 is a graph showing an example of the luminance distribution obtained by the measurement in step S3. The horizontal axis of FIG. 11 indicates the angular position around the optical axis 91. The vertical axis of FIG. 11 represents the luminance at the position overlapping the measurement circle 93 in the image. When the optical axis 91 and the target axis 92 do not coincide with each other, as shown in FIG. 11, the brightness along the measurement circle 93 centered on the optical axis 91 changes in a cycle of about 180 °.

  In this embodiment, the count value in the control unit 60 is set to 0 at the start of positioning. When the measurement of the luminance distribution in step S3 is completed, the control unit 60 increments the count value by 1 (step S4). For example, when the first measurement of the luminance distribution is completed after the rod lens 10 is inserted inside the surface 100 to be inspected, the count value in the control unit 60 is incremented from 0 to 1. Then, the control unit 60 determines whether the count value is larger than a preset threshold value N (step S5). In the present embodiment, the value of the threshold value N is 1, but the value of the threshold value N may be an integer of 2 or more. The value of the threshold N may be set appropriately according to the required number of virtual straight lines described later.

  When the count value is equal to or less than the threshold value N, the second movement instruction section 65 in the control section 60 transmits a control signal to the movement mechanism 50. Thereby, as shown in FIG. 6, the rod lens 10 is moved in the direction orthogonal to the axial direction (step S6). After moving the rod lens 10, the inner surface inspection apparatus 1 again acquires an image (step S2), measures the luminance distribution (step S3), and increments the count value (at the position of the rod lens 10 after the movement). Perform step S4). Then, it is determined whether the count value is larger than the threshold value N (step S5).

  When the count value becomes larger than the threshold value N in step S5, the virtual straight line calculation unit 632 in the control unit 60 obtains a virtual straight line in each of the plurality of images obtained before and after the movement of the rod lens 10 (step S5). S7). The virtual straight line is a straight line that passes through the optical axis 91 of the rod lens 10 and extends at an angle around the optical axis 91 at which the brightness is minimized. For example, when the brightness distribution as shown in FIG. 11 is obtained, a straight line passing through the optical axis 91 and passing through the angles θ1 and θ2 that are 180 ° apart from each other is a virtual straight line. As described above, the brightness in the direction from the optical axis 91 to the target axis 92 becomes low. Therefore, the target axis 92 is estimated to be located near the virtual straight line. In step S7, such a virtual straight line is obtained for each of the acquired images. Thereby, for example, two virtual straight lines L1 and L2 as shown in FIG. 7 are obtained.

  After that, the relative position calculation unit 633 in the control unit 60 calculates the coordinates of the intersection of the two virtual straight lines L1 and L2. Then, the coordinates of the intersection are estimated as the position of the target axis 92 (step S8). When the position of the target axis 92 is estimated, the relative position of the target axis 92 with respect to the optical axis 91 of the rod lens 10 at that time can also be estimated. The third movement instruction unit 66 of the control unit 60 transmits a control signal to the movement mechanism 50 based on the obtained information on the relative position. As a result, the rod lens 10 is moved to the estimated position of the target axis 92 as shown in FIG. 8 (step S9). As a result, the optical axis 91 of the rod lens 10 and the target axis 92 substantially coincide with each other.

  After that, the control unit 60 causes the imaging unit 30 to perform imaging while causing the light source 20 to emit light again at the position of the rod lens 10 after positioning. The image acquired by shooting is input to the image input unit 61 in the control unit 60. The control unit 60 acquires the brightness distribution around the optical axis 91 in the acquired image. FIG. 10 is a diagram showing an example of an image acquired via the rod lens 10 after positioning. When the optical axis 91 and the target axis 92 are substantially coincident with each other, the change in brightness is small around the optical axis 91 in the image, as shown in FIG.

  The determination unit 62 in the control unit 60 determines whether or not the acquired brightness distribution matches a preset allowable condition (step S10). For example, it is determined whether the difference between the maximum value and the minimum value of the acquired brightness is smaller than a preset allowable value. Then, if the difference in brightness is smaller than the allowable value, it is determined that the position of the optical axis 91 and the position of the target axis 92 substantially match, and the positioning of the rod lens 10 is completed. On the other hand, when the difference in brightness is larger than the allowable value, it is determined that the optical axis 91 is not yet sufficiently close to the target axis 92. In that case, the process returns to step S2 and the processes of steps S2 to S10 are executed again.

  As described above, in the inner surface inspection apparatus 1, the control unit 60 automatically determines the position of the rod lens 10 inside the surface 100 to be inspected by utilizing the luminance distribution in the image obtained through the rod lens 10. Fine tune. Therefore, it is not necessary for the operator to manually perform the work of precisely positioning the rod lens 10. As a result, the operator's burden on the operation of the inner surface inspection apparatus 1 can be reduced.

<3. Modification>
Although the exemplary embodiments of the present invention have been described above, the present invention is not limited to the above embodiments.

  In the above embodiment, the threshold N in step S5 is set to 1. Therefore, the movement of the rod lens 10 in step S6 is executed only once. The control unit 60 has measured the luminance distribution for each of the two images acquired before and after one movement to obtain the virtual straight lines L1 and L2. Then, the intersection of the two virtual straight lines L1 and L2 is estimated as the position of the target axis 92. However, the movement of the rod lens 10 in step S6 may be performed twice or more by setting the threshold value N in step S5 to 2 or more. In that case, the luminance distribution may be measured for each of the three or more images acquired before and after the movement to obtain the virtual straight line.

  For example, as shown in FIG. 12, the rod lens 10 may be first moved in the first direction A1 orthogonal to the axial direction and then moved in the second direction A2 orthogonal to the axial direction. In this case, the luminance distribution around the optical axis is acquired in each of the images captured at three time points before the movement of A1, between the movement of A1 and the movement of A2, and after the movement of A2. , Three virtual straight lines L1 to L3 extending at the angle at which the luminance is minimized are obtained. Then, the position of the target axis 92 is estimated inside the triangle connecting the intersections of the three virtual straight lines L1 to L3. For example, the position of the center of gravity of the triangle may be estimated as the position of the target axis 92.

  In this way, if three or more virtual straight lines are obtained and the position of the target axis 92 is estimated inside the polygon connecting the intersections of the virtual straight lines, the target axis 92 is estimated more than when the number of virtual straight lines used for estimation is two. The position of can be estimated more accurately.

  Further, in the example of FIG. 12, after the rod lens 10 is moved in the first direction A1, the rod lens 10 is moved in the second direction A2 different from the first direction A1. With this configuration, the directions of the plurality of virtual straight lines can be changed as compared with the case where the rod lens 10 is moved in the same direction a plurality of times. Therefore, the relative position of the target axis 92 with respect to the optical axis 91 of the rod lens 10 can be estimated more accurately.

  Further, in the above embodiment, the cylindrical rod lens 10 is used. However, instead of the cylindrical rod lens, a cylindrical rod lens extending along the optical axis may be used. Further, in the above embodiment, the tip end surface 12 of the rod lens 10 has a conical shape of 45 ° with respect to the optical axis 91. However, the angle of the tip surface 12 of the rod lens 10 may be another angle depending on the position where the image is captured. The tip surface 12 of the rod lens 10 may have a shape other than the conical shape.

  Moreover, in the above-described embodiment, the personal computer is used as the control unit 60. However, instead of the personal computer, an electric circuit device equipped with an arithmetic processing unit such as a microcomputer having a CPU may be used as the control unit 60.

  It should be noted that the “brightness” in the present invention may be a parameter that reflects the amount of light incident on the imaging unit. Therefore, the “luminance” in the present invention may be a value called density, brightness, illuminance, pixel value, or the like.

  In addition, the detailed shape and structure of the inner surface inspection apparatus may be different from the drawings of the present application. Further, the respective elements appearing in the above-described embodiments and modified examples may be appropriately combined as long as no contradiction occurs.

  INDUSTRIAL APPLICABILITY The present invention can be used for an inner surface inspection device and a positioning method.

1 Inner Surface Inspection Device 10 Rod Lens 11 Base End Surface 12 Tip Surface 20 Light Source 21 Circular Hole 30 Imaging Section 40 Casing 41 Through Hole 50 Moving Mechanism 51 Stage 60 Control Section 61 Image Input Section 62 Judgment Section 63 Estimating Section 64 First Movement Instructing Section 65 Second movement instruction section 66 Third movement instruction section 91 Optical axis 92 Target axis 93 Measurement circle 100 Inspected surface 601 Display section 631 Brightness distribution acquisition section 632 Virtual straight line calculation section 633 Relative position calculation section L1, L2, L3 Virtual straight line

Claims (10)

An inner surface inspection device for inspecting a state of a surface to be inspected, which is a cylindrical inner peripheral surface,
A cylindrical or cylindrical rod lens that extends in the axial direction along the optical axis that is the central axis and is inserted inside the surface to be inspected,
A moving mechanism that moves the rod lens in the axial direction and a direction orthogonal to the axial direction;
An imaging unit that acquires an image via the rod lens,
A control unit for controlling the moving mechanism,
The control unit is
A first movement instructing section for inserting the rod lens inside the surface to be inspected;
A second movement instructing section that moves the rod lens in a direction orthogonal to the axial direction;
In a state in which the rod lens is inserted inside the surface to be inspected, the center axis of the surface to be inspected with respect to the optical axis is based on the luminance distribution around the optical axis in the image acquired by the imaging unit. An estimation unit that estimates the relative position of the target axis,
Based on the relative position, the rod lens have a, and a third movement instruction unit that moves into the target shaft side,
The estimation unit is
In each of the plurality of images acquired before and after the movement of the rod lens by the second movement instruction unit, the luminance distribution around the optical axis is measured, and a virtual straight line extending at an angle at which the luminance is minimized is obtained. ,
An inner surface inspection apparatus that estimates the relative position based on an intersection of a plurality of the virtual straight lines. The inner surface inspection apparatus according to claim 1 , wherein
The estimation unit is
In each of the three or more images acquired before and after the plurality of movements of the rod lens by the second movement instruction unit, the luminance distribution around the optical axis is measured, and the angle at which the luminance is minimized is obtained. Find a virtual straight line that extends,
An inner surface inspection apparatus for estimating the position of the target axis inside a polygon connecting the intersections of three or more virtual straight lines. The inner surface inspection apparatus according to claim 1 or 2 , wherein
The inner surface inspection apparatus, wherein the estimation unit measures the luminance distribution in the image along a circle radially inward of the outer peripheral surface of the rod lens and closer to the outer peripheral surface than the optical axis. The inner surface inspection apparatus according to any one of claims 1 to 3 ,
The second movement instruction unit,
An inner surface inspection apparatus that moves the rod lens in a second direction different from the first direction after moving the rod lens in the first direction. The inner surface inspection apparatus according to any one of claims 1 to 4 ,
The control unit is
The inner surface inspection apparatus further comprising a determination unit that determines whether or not the brightness distribution around the optical axis matches a preset allowable condition after the rod lens is moved by the third movement instruction unit. A cylindrical or cylindrical rod lens extending in the axial direction along the optical axis that is the central axis, inside the surface to be inspected that is a cylindrical inner peripheral surface, a positioning method for positioning.
a) inserting the rod lens inside the surface to be inspected,
b) In a state where the rod lens is inserted inside the surface to be inspected, the rod lens is moved in a direction orthogonal to the axial direction, and the light in the image obtained through the rod lens is obtained. Measuring the brightness distribution around the axis,
c) estimating the relative position of a target axis, which is the central axis of the surface to be inspected, with respect to the optical axis, based on the luminance distribution;
d) on the basis of the relative position, have a, a step of moving the rod lens into the target shaft side
In step b),
Moving the rod lens in a direction orthogonal to the axial direction, in each of the plurality of images acquired before and after the movement, to measure the luminance distribution around the optical axis,
In the step c),
A positioning method that obtains a virtual straight line extending at an angle at which the brightness becomes a minimum and estimates the relative position based on an intersection of the plurality of virtual straight lines. The positioning method according to claim 6 , wherein
In step b),
The rod lens is moved a plurality of times in a direction orthogonal to the axial direction, and a luminance distribution around the optical axis is measured in each of the three or more images acquired before and after the movement, and the step c ) Then,
A positioning method in which a virtual straight line extending at an angle at which the brightness is minimized is obtained, and the position of the target axis is estimated inside a polygon connecting the intersections of three or more virtual straight lines. The positioning method according to claim 6 or 7 , wherein
In step b),
In the image, a positioning method of measuring a luminance distribution along a circle radially inward of an outer peripheral surface of the rod lens and along a circle closer to the outer peripheral surface than the optical axis. The positioning method according to any one of claims 6 to 8 , wherein:
In step b),
A positioning method of moving the rod lens in a first direction and then moving the rod lens in a second direction different from the first direction. The positioning method according to any one of claims 6 to 9 ,
e) The positioning method further comprising the step of, after the step d), determining whether or not the brightness distribution around the optical axis matches a preset allowable condition.